The present invention relates generally to operations performed and equipment utilized in conjunction with a subterranean well and, in an embodiment described herein, more particularly provides diameter based tracking for a window milling system.
In a typical re-entry window milling system, a milling assembly including a lead mill and a follow (or watermelon) mill are deflected laterally relative to a casing or liner string by a milling whipstock. This lateral deflection causes the mills to cut through the casing string to thereby form a window in the casing sidewall. The mills may also be used to drill through cement and/or an earth formation surrounding the casing string, thereby starting a branch wellbore extending outward from the window.
Generally, the lead mill is used to initiate penetration of the casing sidewall, while the follow mill is used to enlarge the window and form it to the desired final shape and dimensions. For reduced resistance to penetration of the casing sidewall, the lead mill may have a smaller diameter than the follow mill, although the mills could have the same diameter. The whipstock deflects both of the mills using the same inclined surface, so that the mills displace along substantially the same path relative to the casing string.
Unfortunately, certain problems arise from use of such prior window milling systems. For example, large bending stresses are experienced when mills having different diameters are guided using the same deflection surface. As another example, substantial wear is experienced when both mills traverse the same surface during the milling operation. Furthermore, prior systems do not take advantage of the unique qualities of the different mills which could be made possible by guiding the mills along respective different paths.
Accordingly, it may be seen that improvements are needed in the art of window milling systems.